As an example of the tire pressure detecting apparatus, a direct-type tire pressure detecting apparatus has been known. In the direct-type tire pressure detecting apparatus, a transmitter is directly fixed to each of wheels equipped with a tire. The transmitter is provided with a sensor, such as a pressure sensor. A vehicle body is equipped with an antenna and a receiver. When the transmitter transmits a detection signal of the sensor, the receiver receives the detection signal through the antenna, and detects the tire pressure of the wheel based on the detection signal.
In such a direct-type tire pressure detecting apparatus, data transmitted from the transmitter includes individual identification (ID) information for identifying whether the transmitted data is associated with a subject vehicle to which the tire pressure detecting apparatus is equipped, and for identifying which wheel the transmitter transmitting the data is fixed to.
To specify the position of the transmitter based on the ID information of the transmitted data, the receiver needs to store beforehand a relationship between the ID information of each transmitter and the position of a corresponding wheel, as a wheel position relationship. When the positions of the wheels are changed by a tire rotation, it is necessary to register the wheel position relationship again.
As a method for automatically registering the wheel position relationship, an initiator method and a zero initiator method are generally known. In the initiator method, initiator (trigger device) is used to permit the transmitter to transmit a frame at a predetermined timing. Although the wheel position can be detected at an early stage by means of an interactive communication, the initiator is necessary. In the zero initiator method, on the other hand, the initiator is not necessary. For example, a wheel position detection by the zero initiator method is described in patent literatures 1-4.